The present invention relates to improved combustion processes and more particularly to an improved method for calcining carbonaceous material such as petroleum coke.
Carbonaceous materials, such as "green" petroleum coke, are typically calcined in a rotary kiln by introducing these materials into the upper end of a slightly inclined kiln and heating the interior thereof to a temperature of approximately 2200.degree.-2600.degree. F. One technique for heating such a kiln is to fire an end burner disposed in a firing hood at the product outlet end or lower end of the kiln and directing a flame longitudinally through the kiln in the direction generally opposed to that of the product being calcined and along the axis of the kiln. This application of heat is effective to evolve or drive off volatile materials from the green carbonaceous material thereby increasing the density of such materials being calcined. As these gases are evolved from the green carbonaceous material, the heat within the kiln is effective to cause a combustion of such volatile gases which in turn releases heat to the kiln interior and enables the firing rate of the end burner to be reduced below a relatively high firing rate. Frequently, combustion of volatiles in the kiln will supply a majority of the heat required to calcine a green carbonaceous material, such as petroleum coke. Typically, during calcination, at least 99.5% of the volatiles of a green carbonaceous material are evolved therefrom and are combusted, as virtually complete volatilization is required in order to produce a calcined product of suitable quality. With regard to petroleum coke, calcination will typically be effective to increase the density thereof from approximately 1.6 to about 2.6 g/cm.sup.3. This enables the resulting petroleum coke to be utilized for several purposes, including use as a fuel.
The burners employed must burn fuels, such as natural gas or fuel oil. Costs of these fuels have increased substantially, thus increasing the cost of the calcining operation.
Several proposed methods for reducing the amount of fuel required to operate a calcining kiln have been proposed. Thus, U.S. Pat. Nos. 2,813,822 and 3,888,621 control airflow within the kiln to improve combustion of the volatile gases. In U.S. Pat. No. 4,354,829, oxygen is introduced to the kiln-mounted blowers of U.S. Pat. No. 2,813,822, in an attempt to further improve the combustion process.
While the known processes have reduced fuel usage, there remains a need for even greater reductions in fuel consumption. It is thus a primary objective of the present invention to substantially reduce combustion fuel usage for the life of a kiln and to eliminate combustion fuel usage during normal operations of the kiln.
It is also a primary objection of the present invention to replace the relatively expensive combustion fuel with relatively inexpensive oxygen during normal operations of the kiln.
It is another primary objective of the present invention to employ a portion of the carbonaceous material being calcined as the fuel for the kiln.
It is yet another objective of the present invention to maintain or improve total material recovery by substantially reducing total gas flows in the kiln.